Flip chip technology, first introduced in the 1960's by IBM as the controlled collapse chip connection (C4) process, offers a viable and proven alternative to standard assembly technologies for products requiring enhanced performance. Flip chip is not a specific package (like SOIC), or even a package type (like BGA). Rather, flip chip generally describes the method of electrically connecting an integrated circuit (IC) die, also referred to as a chip, to a package carrier. The package carrier, either substrate or leadframe, then provides the connection from the die to the exterior of the package. In “standard” IC packaging, the interconnection between the die and the carrier is made using bond wires, which exhibit disadvantages, particularly in high-frequency applications (e.g., about one gigahertz and above).
Flip chip has become popular primarily because it offers good electrical performance, small package size, and the capability of handling a relatively large number of input/output (I/O) connections. Early flip chip processing employed solder bumps formed on chip I/O pads. These solder bumps align with corresponding pad sites on the substrate. The solder bumped die is attached to the substrate by a solder reflow process, very similar to the process used to attach BGA balls to the package exterior. After the die is attached, underfill is added between the die and the substrate to control the stress in the solder joints caused by the difference in thermal expansion between the silicon die and the carrier. Once cured, the underfill absorbs the stress, reducing the strain on the solder bumps, greatly increasing the life of the finished package. The die attach and underfill steps are the basics of flip chip interconnect.
As flip chip technology is extended for use with chips having higher pin counts, smaller line-pitch and larger size, such as, for example, microprocessor chips and chipsets, the size of the solder bumps on the chip decreases, and thus the amount of tolerance to substrate warpage in the chip decreases accordingly. Consequently, reliability of standard flip chip technology is often unacceptable when used in such applications.